The sensor is black and white. This lets them collect the most accurate information about how much light hits each part of the sensor. To take a color image they put a color filter in front of the lens, then they can recolor the black and white image according to the color of the filter. If they take multiple pictures with different filters, they can reconstruct a color image. http://areo.info/mer/
They have sent back at least one color photo, which has been released [1] as a test that the cameras are working. They're still getting everything operational (and I believe they have a software upgrade to do yet) before the major operations get underway. The black and white cameras don't need to be deployed and calibrated like the color/high-res cameras do.
The B&W cameras are primarily used for navigation and hazard avoidance. The rover is equipped with color cameras, and they will get stood up in time. For now they are starting with the basic pieces and gradually bringing more online.
